C-X-C chemokine receptor type 4 (CXCR4) is an alpha-chemokine receptor that binds the ligand SDF-1 and transmits intracellular signals through several different pathways resulting in an increase in calcium and/or a decrease in cAMP levels. The extracellular regions of CXCR4 are composed of 4 discontinuous segments (N-terminal domain, extracellular loop (ECL) 1, ECL2, and ECL3), comprising a total of 91 amino acids.